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# Solving 3rd order ODE

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Kevin Tran on 10 May 2020
Commented: Star Strider on 11 May 2020
Hi guys, I need to solve this 3rd order ODE:
?⃛+2??̈+3??̇+4??=2, t≥0, ?̈(0)=?̇(0)=?(0)=0
This is my script:
[t,y] = ode45(@diffeq3,[0 50],[0;-1;1]);
plot(t,y)
function dy = diffeq3(t,y)
if t >= 0
dy(3) = 0
dy(2) = 0
dy(1) = 0
end
dy = zeros(3,1);
dy(1) = y(2);
dy(2) = y(3);
dy(3) = 2 - 2*y(3) - 3*y(2) - 4*y(1);
end
Did I do it right?
I got two questions, I got three curves which on is which?
And did I define t≥0, ?̈(0)=?̇(0)=?(0)=0 correct? With:
if t >= 0
dy(3) = 0
dy(2) = 0
dy(1) = 0
Thanks in advance for the help!
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### Accepted Answer

Star Strider on 10 May 2020
There appears to be a factor of ‘y(1)’ missing in the last 3 terms of the ‘dy(3)’ equation. (I gave your differential equation as posted to the odeToVectorField function to be certain.)
Also, the if block is not necessary. It does nothing, and produces slower code.
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Star Strider on 11 May 2020
As always, my pleasure!

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